ELearning

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E-Learning ?? ??????
Diversity
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Outline

• Diversity
• Combining Techniques
• Conclusions

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Diversity
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Diversity(1/13)

• Overcoming Channel Impairments

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Diversity(2/13)

• Diversity
• ?Aim Reduce effects of fast fading
• ?Concept
• Multiple branches, independent fading
• Process branches to reduce fading probability
• ?If probability of a deep fade on one channel is
  p, probability on N channel pN .
• ?e.g. 10 chance of losing contact for one
  channel becomes 0.130.0010.1 with 3 channels

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Diversity(3/13)

• Example Select Largest Signal

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Diversity(4/13)

• Requirements for Diversity
• ?Multiple branches
• ?Low correlation between branches
• ?Correlation defined as
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  i.e. for Rayleigh
• ?Similar mean powers
• ?Efficient combiner

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Diversity(5/13)

• Types of Diversity
• ?Space Diversity
• Horizontal
• Vertical
• ?Polarisation Diversity
• ?Time Diversity
• ?Frequency Diversity

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Diversity(6/13)

• Space Diversity
• ?Large antenna spacing or large scatterer spacing
  produce large path length differences
• ?Hence multipath will combine differently at each
  antenna

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Diversity(7/13)

• Analysis of Space Diversity
• ?Phase difference
• ?Signals from one scatterer
• ?Signals from ns scatterer
• ?Correlation
• ?Evaluate expectation

Angle-of-arrival PDF
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Diversity(8/13)

• Horizontal Space Diversity at Mobile
• ?Take p(?) 1/2p
• ?Then

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Diversity(9/13)

• Vertical Space Diversity at Mobile
• ?Restricted vertical angle spread, so greater
  separation
• needed in vertical direction

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Diversity(10/13)

• Polarisation Diversity
• ?Scattering shifts and decorrelates polarisation
• ?Advantage Very compact
• ?Disadvantage Unequal branch powers - less
  diversity gain

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Diversity(11/13)

• Polarisation Diversity Model
• ?Orthogonal field components assumed uncorrelated
• ?Cross Polar ratio
• ?Hence correlation
• ?Combined space and polarisation diversity

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Diversity(12/13)

• Time Diversity
• ?Retransmit with Time Separation
• ?Advantage Need only one receiver
• ?Disadvantage Wastes bandwidth, adds delay

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Diversity(13/13)

• Frequency Diversity
• ?Wideband Channel
• ?Simultaneous Transmission
• ?Wastes power and bandwidth
• ?Equalisers

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Combining Techniques
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Combining Techniques(1/17)

• Combining Techniques
• ?Selection Combining
• ?Switched Combining
• ?Equal Gain Combining
• ?Maximal Ratio Combining

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Combining Techniques(2/17)

• Generic Combining Architecture

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Combining Techniques(3/17)

• Selection Combining

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Combining Techniques(4/17)

• Performance of Selection Combining
• ?Assuming all branches have same noise power
• ?Instantaneous SNR
• ?If all branches independent Rayleigh
• ?For deep fades
• ?Hence (10/N) dB per decade (cf. 10dB per decade
  for standard one branch Rayleigh)

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Combining Techniques(5/17)

• Performance of Selection Combining

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Combining Techniques(6/17)

• Effect of Varying Branch Mean Powers

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Combining Techniques(7/17)

• Switched Combining
• ?Avoids multiple receivers
• ?Switch and stay strategy
• ?Must set appropriate threshold relative to mean
  level
• ?Performance always worse than selection combining

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Combining Techniques(8/17)

• Equal Gain Combining
• ?Make use of energy in all branches

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Combining Techniques(9/17)

• Equal Gain Combining Performance
• ?Received signals
• ?Combiner output
• ?SNR

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Combining Techniques(10/17)

• Maximal Ratio Combining
• ?Weight branches to maximise SNR

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Combining Techniques(11/17)

• MRC Performance
• ?Optimum weights are
• ?Combiner output
• ?SNR (uncorrelated branches)
• ?2-branch fade probability (correlated case)

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Combining Techniques(12/17)

• Varying Branch Correlations

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Combining Techniques(13/17)

• Effect of Non-zero correlation on MRC

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Combining Techniques(14/17)

• SNR for BPSK with MRC

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Combining Techniques(15/17)

• Comparison of Combining Techniques

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Combining Techniques(16/17)

• Interleaving

Block interleaver where source bits are read
into columns and read out as n-bit rows.
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Combining Techniques(17/17)

• Channel Coding
• ?Channel coding protects digital data from errors
  by selectively introducing redundancies in the
  transmitted data.
• ?Error detection codes used to detection errors
• ?Error correction codes detected and correct
  errors
• block codes Hamming codes, Hadamard codes,
  Cyclic Codes, BCH codes, Reed-Solomon Codes
• convolutional codes
• ?Trellis Coded Modulation Ungerboeck, 1987 Com.
  Mag. Feb., 1987
• Combine both coding and modulation to achieve
  significant coding gains without compromising
  bandwidth efficiency

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Conclusions
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Conclusions(1/1)

• Conclusions
• ? Diversity effectively overcomes multipath
  fading
• ? Large gains (e.g. 10dB) relatively easy to
  provide
• ? Costs in extra hardware (antennas, receivers,
  combiner)
• ? Requires low correlations and significant mean
  powers

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The End